The present invention relates to apparatus and methods for determining the distance between an investigating device and the surface of a medium. More specifically, the invention may be used to determine the internal size of a subsurface open or uncased borehole or, more specifically, to determine the standoff distance between a borehole wall and a tool in the borehole such as a wireline logging sonde or a measurement while drilling (MWD) drill collar.
One of the parameters useful in the investigation of a subsurface geological formation and the borehole penetrating it is the borehole diameter. In the past, with wireline logging, it has been satisfactory to determine the borehole size by a physical device, such as by means of a borehole caliper having at least one extendible arm that is brought to bear against the borehole wall. U.S. Pat. No. 2,719,363 discloses one attempt to perform borehole size measurements from a drill string. In this approach, however, the caliper would only be enabled during the process of tripping out of the well in a manner not too dis-similar from the wireline. In the past, it has not been considered possible to make borehole size determinations with mechanical devices while drilling (as opposed to while tripping) due to the rather severe physical environment to which the measurement while drilling instruments are subjected. So severe is this environment, that one would not expect a physically extendible arm that reaches out and contacts the borehole wall while the drilling process is in progress, to survive.
In another approach, illustrated by U.S. Pat. Nos. 4,661,933, and 4,665,511 an ultrasonic pulse echo technique has been proposed while in yet another approach described in U.S. Pat. No. 4,791,797, measurements from a gamma ray density investigation and a neutron porosity investigation are combined with knowledge of the lithology to obtain a caliper measurement.
Also known are techniques which inspect the interior of a metallic pipe such as are described in U.S. Pat. Nos. 4,292,588, 4,292,589, 4,546,314, and 4,546,315. These and similar metallic casing inspection techniques are discussed in the following literature references:
SPE Paper No 115 presented at the 36th Annual Fall Meeting of the SPE-AIME, Oct. 8-11, 1961--"Evaluation of Casing Corrosion by Means of An Electromagnetic Casing Inspection Well Logging Device", by: Stanley G. Stroud and Charles A. Fuller
SPE Paper No. 664 presented at the 38th Annual Fall Meeting of the SPE-AIME, Nov. 4-5, 1965--"A Report on Field Results of the Electromagnetic Casing Inspection Log", by: J. M. Edwards and S. G. Stroud
SPE Paper No. 1327 presented at the 36th Annual California Regional Fall Meeting of the SPE-AIME, Nov. 4-5, 1965--"New Electronic Casing Caliper Log Introduced for Corrosion Detection", by: J. M. Edwards and S. G. Stroud
Paper presented at the 17th Annual Conference of The National Association of Corrosion Engineers, Mar. 13-17, 1961--"The Casing Inspection Tool--An Instrument for the In-Situ Detection of External Casing Corrosion in Oil Wells", by: T. R. Schmidt
Recent developments in the industry of subsurface investigations, however, have begun to require that the borehole diameter measurements be performed while the well is being drilled. In many instances it is desireable to know the size of the borehole in order to be able to assess such quantities as borehole size and tool standoff which may have a number of deleterious effects on the various investigations. Additionally, knowledge of the size of the borehole contemporaneous with drilling would be of substantial assistance to the driller as it would give him an indication of the stability of the borehole.
At least one ultrasonic pulse echo investigation has been proposed for implementation during the drilling process. The above mentioned U.S. Pat. No. 4,665,511 describes an ultrasonic device intended for this purpose. Ultrasonics are, however, faced with a number of difficulties such as signal scatter from the drill cuttings in the borehole, severe signal attenuation in heavy drilling muds and a backscattered signal that "misses" when the face of the transceiver is not parallel to the tangent of the surface of the hole.
Thus, a reliable, accurate determination of the diameter of a borehole while the borehole is being drilled is sorely needed in the field of subsurface drilling and exploration.